Sonic and especially ultrasonic energy is being increasingly utilized in the nondestructive testing or inspection of parts for defects. In a typical form of such apparatus a quantity of water on the part to be tested serves as a coupling means for sonic energy generated by a relatively remotely located transducer. It has been found that even the presence of a very small amount of surface irregularity in the column causes sonic reflections which substantially impair operation of the apparatus by impeding flow of the sonic energy through the water to the test piece. It is therefore a desideratum to provide water coupling between the sonic transducer and the surface to be inspected which is as free from surface irregularities as possible.
In a frequently encountered testing apparatus particularly for use in the scanning inspection of large sheetlike surfaces by sonic means, a jet-like stream of water is emitted toward the surface to be tested and along which jet the sonic energy passes. U.S. Pat. No. 4,004,736 granted Jan. 25, 1977 discloses an ultrasonic water jet for use in this general type of apparatus and which is stated as providing a 4 to 6 inch stream of water which is unbroken and free of bubbles.
Both the patented device as well as other known devices for producing water jets in the art have not been found capable of producing a jet or stream free from surface irregularities extending for more than about 6 inches, and this fact sets an inherent restriction on present sonic nondestructive test equipment. That is, either the jet nozzles have to be located close enough to the test piece so that the stream will be in its pure homogeneous state, which is not always possible, or the degradation in the water stream associated with greater lengths of the stream must be compensated for in some manner.